The present invention relates to heating of implanted devices. More specifically, the present invention relates to non-invasive heating of implanted vascular treatment devices, such as stents.
Current vascular treatment devices, such as stents, suffer from a number of disadvantages. Among the primary disadvantages is restenosis. A number of different methods have been proposed to address this complication. Among those methods include heating the cells proximate the stent to induce cell apoptosis.
The methods used to heat the cells vary. However, one method is described in Diamantopolous, Langenhove, Foley, Feyter, Non-Invasive Heating of Implanted Arterial Stents In Vivo: A Novel New Method To Prevent Restenosis, Feasibility and Safety. The method of heating the stent proposed by Diamantopolous et al. is based on inherent properties of metals when placed inside an alternating electromagnetic field. Because of the retentively of the metallic stent used in Diamantopolous et al., the stent forms a magnetic circuit. As the magnetizing force of an alternating magnetic field periodically changes, the magnetic flux inside the stent lags, resulting in power loss in the stent. At least a portion of the power loss, of course, manifests itself as heat.
In Diamantopoulos et al., a high frequency alternating magnetic field was generated based on control signals from a personal computer. In a human coronary artery model, donor blood was pressurized to achieve a desired flow rate through the model. The stents were heated to 60° C. Diamantopoulos et al. also mentioned that maintenance of the stent temperature at levels of 43-45° C. would be feasible by power-algorithm and magnetic feedback techniques. Diamantopoulos et al. also verified that Nitinol stents could be heated to accomplish remote expansion.
One of the primary disadvantages with the technique mentioned by Diamantopoulos et al. is that the heating in the stent is dependent upon the change of magnetic flux through the stent. This, in turn, is dependent on the alignment and positioning of the stent relative to the magnetic field lines generated in the applied magnetic field. If the magnetic field and the stent are not aligned properly, then little or no heating effect will be obtained.
Another disadvantage with the Diamantopoulos et al. technique is that the stent temperature is raised by the resistance to the electric currents induced by the change in magnetic flux. That temperature rise is therefore not distributed homogeneously throughout the stent. This is because the stent is not a homogeneous tube, and therefore some points within the stent will become hotspots, which can damage the arterial wall.
A further disadvantage of the Diamantopoulos et al. technique is its lack of flexibility. In other words, if the stent is aligned with the magnetic field, the entire stent will heat. If it is not aligned with the magnetic field, then the stent will not heat. In either case, there is no mechanism by which only portions of the stent can be heated while retaining other portions of the stent substantially unheated under the influence of the applied magnetic field.